Referring to FIGS. 21 and 22, the rod 230 may extend in the lateral direction. The rod 230 may extend through the second part 222b, the first wing 210b, the second wing 210c and the third part 222c in that order. For example, the rod 230 may be composed of a long bolt, and may be engaged with a nut 230N. Here, the second part 222b, the first wing 210b, the second wing 210c and the third part 222c may be disposed between the head provided at one end of the rod 230 and the nut 230N provided adjacent to the other end of the rod 230. A first washer w1 may be disposed between the head of the rod 230 and the second part 222b, and a second washer w2 may be disposed between the nut 230N and the third part 222c.